A crankshaft is a principal component of a reciprocating engine, which obtains power by converting reciprocating motion of pistons to rotary motion. Generally, there are two types of crankshafts: those that are manufactured by forging and those that are manufactured by casting. For engines of automobiles such as passenger cars, freight cars, and specialized work vehicles, particularly, for multiple cylinder engines having two or more cylinders, their crankshafts are required to have high strength and stiffness and therefore forged crankshafts more capable of meeting the request are widely used. Further, forged crankshafts are also used in multiple cylinder engines of motorcycles, agricultural machines, marine vessels, and the like.
In general, forged crankshafts for multiple cylinder engines are manufactured by using, as a starting material, a billet having a circular or square cross section and having a constant cross-sectional area along the entire length, and subjecting the billet to the steps of preforming, die forging, trimming, and coining in the mentioned order (for example, refer to Patent Literatures 1 to 3). The preforming step includes roll forming and bending, and the die forging step includes block forging and finish forging.
FIG. 1 is a schematic diagram illustrating a typical conventional process for manufacturing a forged crankshaft. FIG. 2 is a schematic diagram illustrating an example of a conventional forged crankshaft, FIG. 2(a) is a plan view, FIG. 2(b) is a side view, and FIG. 2(c) is a radial cross-sectional view of a journal representative of a shaft part.
A crankshaft 1 illustrated by examples in FIGS. 1 and 2 is intended to be mounted in a 4-cylinder engine. It is a 4-cylinder 8-counterweight crankshaft that includes: five journals J1 to J5; four crank pins P1 to P4; a front part Fr, a flange Fl, and eight crank arms (hereinafter simply referred to as “arms”) A1 to A8 that connect the journals J1 to J5 and the crank pins P1 to P4 to one another, wherein each of the eight arms A1 to A8 has a balance weight. Hereinafter, when the journals J1 to J5, the crank pins P1 to P4, and the arms A1 to A8 are each collectively referred to, a reference character “J” is used for the journals, a reference character “P” for the crank pins, and a reference character “A” for the arms. Among these portions, the journals J, the crank pins P, the front part Fr, and the flange Fl are all column-shaped shaft parts.
According to the manufacturing method shown in FIG. 1, the forged crankshaft 1 is manufactured in the following manner. Firstly, a billet 2 shown in FIG. 1(a), which has been previously cut to a predetermined length, is heated by an induction heater or a gas atmosphere furnace and then is subjected to roll forming. In the roll forming step, for example, the billet 2 is rolled and reduced in cross section by grooved rolls to distribute its volume in the longitudinal direction; thereby, a rolled blank 103, which is an intermediate material, is formed (see FIG. 1(b)). Next, in the bending step, the rolled blank 103 obtained by roll forming is partially pressed in a press in a direction perpendicular to the longitudinal direction to distribute its volume; thereby, a bent blank 104, which is a secondary intermediate material, is formed (see FIG. 1(c)).
Then, in the block forging step, the bent blank 104 obtained by bending is press forged with a pair of upper and lower dies; thereby, a forged blank 105 having a general shape of a crankshaft (forged final product) is formed (see FIG. 1(d)). Thereafter, in the finish forging step, the block forged blank 105 obtained by block forging is provided and processed by press forging with a pair of upper and lower dies; thereby, a forged blank 106 having a shape coinciding with the shape of the crankshaft is formed (see FIG. 1(e)). In the block forging and finish forging steps, excess material flows out as a flash from between parting surfaces of the dies that oppose each other. Therefore, each of the block forged blank 105 and the finish forged blank 106 has a large flash 105a, 106a around the formed shape of the crankshaft.
In the trimming step, the finish forged blank 106 with the flash 106a, obtained by finish forging, is held by dies from above and below and the flash 106a is trimmed by a cutting die. In this manner, the forged crankshaft 1 is obtained as shown in FIG. 1(f) and FIG. 2. In the coining step, principal parts of the forged crankshaft 1, from which the flash has been removed, e.g., shaft parts such as the journals J, the crank pins P, the front part Fr, and the flange Fl, and in some cases the arms A, are slightly pressed with dies from above and below and are corrected to a desired size and shape. Thus, the forged crankshaft 1 is manufactured.
The manufacturing process shown in FIG. 1 is applicable not only to a 4-cylinder 8-counterweight crankshaft as illustrated by an example, but also to a 4-cylinder 4-counterweight crankshaft in which, among 8 arms A, the leading first arm A1, the trailing eighth arm A8, and the two central, fourth and fifth arms A4, A5 have balance weights. Also, the same manufacturing process can be applied to crankshafts that are to be mounted in a 3-cylinder engine, an inline 6-cylinder engine, a V-type 6-cylinder engine, an 8-cylinder engine, and the like. It is noted that, when adjustment of the placement angle of the crank pins is necessary, a step of twisting is added after the trimming step.
As shown in FIG. 2, a flash is generated in the aforementioned die forging step and the flash is removed in the trimming step; therefore, a flash line 107 as a trace after the trimming step emerges around the entire circumference of the forged crankshaft 1. A radial cross-sectional shape of the journals J, the crank pins P, the front part Fr, and the flange Fl which are shaft parts is a substantially circular shape of the combination of semicircles symmetric to each other in a state where the flash line 107 is positioned between the semicircular portions (see FIG. 2(c)). The semicircular portions are respectively formed by semicircular die impressions of the pair of upper and lower dies in the finish forging step and the position of the flash line 107 corresponds to the positions of the parting surfaces of the upper and lower dies (see a dashed line of FIG. 2(c)).
Such forged crankshaft 1 is subjected to various machining processes, and may be subjected to a heat treatment in order to function as a crankshaft to be mounted in an engine. For example, an outer circumference of each of the journals J, the crank pins P, the front part Fr, and the flange Fl which are shaft parts is processed by cutting to have a predetermined outer diameter (see a two-dot chain line of FIG. 2(c)). In particular, the outer circumferences of the journals J and the crank pins P are further processed by grinding and are finished to have predetermined outer diameters and surface roughness. The journals, each outer circumference of which is processed by machining are supported via a slide bearing bush by an engine block. Likewise, the crank pins are connected by slide bearing bushes to end portions of connecting rods to which pistons are connected.
Further, the journals and the crank pins slide with the slide bearing bushes, therefore being required to have wear resistance. Therefore, the outer circumferences of the journals and the crank pins are often subjected to induction heating (IH) after cutting and before grinding.